How to make textures in Blender without any flat work with a painting program? (3D painting)
I think you can use a Texture Paint mode and paint the textures using a Stencil option. 

Add a new material to your object. Join all the meshes using Ctrl+J. Unwrap the model using Smart UV Project.


In Uv Editor add a new image texture and name it. Then save it (Image-->Save as Image).


In CYCLES Setup the nodes as pictured below. In the Image Texture node open up the newly created image.


In BLENDER INTERNAL RENDER add a new Paint Slot-->Diffuse Color in a Tools bookmark of a Tool Shelf (T).


Enter the Texture Paint mode and set up the shading type to Texture. Add a new texture in the panel on the left, then go to the Texture header on the right, select Brush and open a texture, which will be used as a stencil for painting.


Enable a Stencil option in a panel on the left. Press:

-LMB to paint through the stencil

-RMB to move it

-Shift+RMB to make the stencil bigger and smaller

-Ctrl+RMB to rotate it.


You may also uncheck the Occlude, Cull and Normal boxes to paint on the invisible parts of the mesh.


Save your image when you're done with painting.